Thin Films Yield Pollution Sensor

Hank Hogan

A team of English and Irish researchers has developed a sensor that uses light to sniff out waterborne pollutants. The device, which is manufactured using standard semiconductor processing techniques, could be mass-produced for less than $6 each.

Thin films, each sensitive to a different pollutant, form the basis of the detector. The researchers produced 71 types of film, nearly half of which were previously unreported. Team members at Sheffield University in the UK developed the thin films, and the researchers at the National Microelectronics Research Centre at University College built the device. Brendan O'Neill, operations director of the center's central fabrication facility, said they used a variety of deposition techniques, with excellent results.

The sensor works by interferometry. A waveguide that splits and then rejoins is etched into silicon, with a thin film deposited in the channel that is covered by a layer of silicon dioxide. One arm of the waveguide has a window cut through this layer. A laser and fiber optics send a 1.3-µm beam through the structure.

When the sensor is exposed to a pollutant, the chemical binds to the thin film uncovered by the window. This changes the optical properties of the waveguide and creates a shift in the fringe pattern from the interferometer. The magnitude of the shift reveals the concentration of the pollutant. Laboratory tests found that the device is sensitive to 50 ppm.

Funding Needed

Standard techniques for monitoring pollutants are effective for electroactive gases such as nitrous oxide, carbon monoxide and chlorine. However, they cannot detect electroneutral organic compounds such as benzene, toluene and xylene, which by law must be monitored as well. The sensor offers a cheap and disposable solution, O'Neill said.

Although the device has been demonstrated in the lab, further funding is needed to develop a commercially viable sensor.